Because your body’s internal clock is just as good, if not better, than the contraption shrieking atop your nightstand.

At the center of your brain, a clump of nerves—called the suprachiasmatic nucleus—oversees your body’s clock: the circadian rhythm. It determines when you feel sleepy and when you feel bright-eyed. It controls your blood pressure, your body temperature, and your sense of time. It turns your body into a finely tuned machine.

That machine happens to love predictability. Your body is most efficient when there’s a routine to follow. So if you hit the hay the same time each night and awake the same time each morning, your body locks that behavior in. And that’s where things get sciency.

Beat the clock!

Your sleep-wake cycle is regulated by a protein called PER. The protein level rises and falls each day, peaking in the evening and plummeting at night. When PER levels are low, your blood pressure drops, heart rate slows, and thinking becomes foggier. You get sleepy.

If you follow a diligent sleep routine—waking up the same time every day—your body learns to increase your PER levels in time for your alarm. About an hour before you’re supposed to wake up, PER levels rise (along with your body temperature and blood pressure). To prepare for the stress of waking, your body releases a cocktail of stress hormones, like cortisol. Gradually, your sleep becomes lighter and lighter.

And that’s why you wake up before your alarm. Your body hates your alarm clock. It’s jarring. It’s stressful. And it ruins all that hard work. It defeats the purpose of gradually waking up. So, to avoid being interrupted, your body does something amazing: It starts increasing PER and stress hormones earlier in the night. Your body gets a head start so the waking process isn’t cut short. It’s so precise that your eyelids open minutes—maybe even seconds—before the alarm goes off.

You snooze, you lose

There’s evidence you can will yourself to wake on time, too. Sleep scientists at Germany’s University of Lubeck asked 15 volunteers to sleep in their lab for three nights. One night, the group was told they’d be woken at 6 a.m., while on other nights the group was told they’d be woken at 9 a.m..

But the researchers lied—they woke the volunteers at 6 a.m anyway. And the results were startling. The days when sleepers were told they’d wake up early, their stress hormones increased at 4:30 a.m., as if they were anticipating an early morning. When the sleepers were told they’d wake up at 9 a.m., their stress hormones didn’t increase—and they woke up groggier. “Our bodies, in other words, note the time we hope to begin our day and gradually prepare us for consciousness,” writes Jeff Howe at Psychology Today.

Incidentally, if you don’t wake before your alarm, you probably aren’t getting enough sleep—or you aren’t sleeping on a consistent schedule. Waking up at different times on weekdays and weekends can quickly throw your clock out of whack. Without any consistency, your body may not know when to get up. So when your alarm starts screaming, you feel dazed and grumpy.

Enter the snooze button. Since your body’s gone through all that work to rise gradually, a quick nap sends your internal clock spinning in the wrong direction. All the hormones that help you fall asleep meddle with the hormones that help you wake up. Your body gets confused. You feel groggier. And with each slap of the snooze, it gets worse. The snooze, it seems, is the worst way to start your day.